Microprocessor-accessible memory devices have traditionally been classified as either non-volatile or volatile memory devices. Non-volatile memory devices are capable of retaining stored information even when power to the memory device is turned off. Traditionally, however, non-volatile memory devices occupy a large amount of space and consume large quantities of power, making these devices unsuitable for use in portable devices or as substitutes for frequently-accessed volatile memory devices. On the other hand, volatile memory devices tend to provide greater storage capability and programming options than non-volatile memory devices. Volatile memory devices also generally consume less power than non-volatile devices. However, volatile memory devices require a continuous power supply in order to retain stored memory content.
Research and development of commercially viable memory devices that are randomly accessed, have relatively low power consumption, and are non-volatile is ongoing. One ongoing area of research is in resistive memory cells where resistance states can be programmably changed. One avenue of research relates to devices that store data in memory cells by structurally or chemically changing a physical property of the memory cells in response to applied programming voltages, which in turn change cell resistance. Examples of variable resistance memory devices being investigated include memories using variable resistance polymers, perovskite, doped amorphous silicon, phase-changing glasses, and doped chalcogenide glass, among others.
FIG. 1 shows a basic composition of a variable resistance memory cell such as a phase change memory cell 1 constructed over a substrate 2, having a variable resistance material 4 formed between a bottom electrode 3 and a top electrode 5. One type of variable resistance material may be amorphous silicon doped with V, Co, Ni, Pd, Fe and Mn as disclosed in U.S. Pat. No. 5,541,869 to Rose et al. Another type of variable resistance material may include perovskite materials such as Pr(1-x)CaxMnO3 (PCMO), La(1-xCaxMnO3 (LCMO), LaSrMnO3 (LSMO), GdBaCoxOy (GBCO) as disclosed in U.S. Pat. No. 6,473,332 to Ignatiev et al. Still another type of variable resistance material may be a doped chalcogenide glass of the formula AxBy, where B is selected from among S, Se and Te and mixtures thereof, and where A includes at least one element from Group III-A (B, Al, Ga, In, Tl), Group IV-A (C, Si, Ge, Sn, Pb), Group V-A (N, P, As, Sb, Bi), or Group VII-A (F, Cl, Br, I, At) of the periodic table, and with the dopant being selected from among the noble metals and transition metals, including Ag, Au, Pt, Cu, Cd, Ir, Ru, Co, Cr, Mn or Ni, as disclosed in U.S. Pat. Nos. 6,881,623 and 6,888,155 to Campbell et al. and Campbell, respectively. Yet another type of variable resistance material includes a carbon-polymer film comprising carbon black particulates or graphite, for example, mixed into a plastic polymer, such as that disclosed in U.S. Pat. No. 6,072,716 to Jacobson et al. The material used to form the illustrated electrodes 3, 5 can be selected from a variety of conductive materials, such as tungsten, nickel, tantalum, titanium, titanium nitride, aluminum, platinum, or silver, among others.
Much research has focused on memory devices using memory elements composed of phase changing chalcogenides as the resistance variable material. Chalcogenides are alloys of Group VI elements of the periodic table, such as Te or Se. A specific chalcogenide currently used in rewriteable compact discs (“CD-RW”) is Ge2Sb2Te5. In addition to having valuable optical properties that are utilized in CD-RW discs, Ge2Sb2Te5 also has desirable physical properties as a variable resistance material. Various combinations of Ge, Sb and Te may be used as variable resistance materials and which are herein collectively referred to as “GST” materials. Specifically, GST materials can change structural phases between an amorphous phase and two crystalline phases. The resistance of the amorphous phase (“a-GST”) and the resistances of cubic and hexagonal crystalline phases (“c-GST” and “h-GST,” respectively) can differ significantly. The resistance of amorphous GST is greater than the resistances of either cubic GST or hexagonal GST, whose resistances are similar to each other. Thus, in comparing the resistances of the various phases of GST, GST may be considered a two-state material (amorphous GST and crystalline GST), with each state having a different resistance that can be equated with a corresponding binary state. A variable resistance material such as GST whose resistance changes according to its material phase is referred to as a phase change material.
The transition from one GST phase to another occurs in response to temperature changes of the GST material. The temperature changes, i.e., heating and cooling, can be caused by passing differing strengths of current through the GST material. The GST material is placed in a crystalline state by passing a crystallizing current through the GST material, thus warming the GST material to a temperature wherein a crystalline structure may grow. A stronger melting current is used to melt the GST material for subsequent cooling to an amorphous state. As the typical phase change memory cell uses the crystalline state to represent a binary 1 and the amorphous state to represent a binary 0, the crystallizing current is referred to as a write or set current ISET and the melting current is referred to as an erase or reset current IRST. One skilled in the art will understand, however, that the assignment of GST states to binary values may be switched if desired.
Phase change memory cells known in the prior art typically have two stable resistance states, corresponding to the binary 0 and 1. Thus, a conventional two-state phase change memory cell can store one bit of information. Phase change memory cells with more than two stable resistance states are desirable because they would allow each cell to store more than one bit of information, thereby increasing memory storage capacity without significantly increasing storage device size or power consumption.
Researchers in China have proposed one such multi-state phase change memory cell using stacked calcogenide films as storage media. See Y. Lai, et al., Stacked chalcogenide layers used as multi-stage storage medium for phase change memory, Appl. Phys. A 84, 21-25 (2006). As shown in FIG. 2A, the proposed multi-state phase change memory cell 200 comprises a bottom electrode 201, a pure GST layer 202, a tungsten layer 203, a silicon-doped GST layer 204, and a top electrode 205. This proposed phase change memory cell 200 provides three relatively stable resistance states (1), (2), and (3), as illustrated by FIG. 2B. Implementing three-state logic with cell 200 is difficult. There is a need for a multiple bit phase change memory cell which is easily implemented and which provides more than three stable resistance states.